The present invention relates generally to sputtering techniques and to the sputter deposition of dielectric films on substrates, and specifically relates to an apparatus and method for maintaining a stable, continuous plasma during deposition of dielectric films.
Sputter deposition is used extensively within the semiconductor industry to deposit thin metallic material and nonmetallic material films or layers onto a semiconductor substrate. Cathode sputter deposition, in particular, is widely utilized and involves the use of a negatively-biased target of the desired sputtering material which is to be deposited onto the surface of a substrate. The target is supported in the processing space of a vacuumed sputter deposition chamber in a position generally opposite and facing the surface of the substrate to be sputter coated with a layer of the target material.
A negative electric potential is applied to the target through a cathode support which is coupled to an appropriate plasma power source. The negatively-biased target produces an electric field proximate the target which causes electrons to be emitted from the target surface toward a remote anode, such as the internal metal wall surfaces of the sputter chamber. The metal sputter chamber is coupled to a source of ground potential or a xe2x80x9cground,xe2x80x9d and the internal wall surfaces thus act as a ground reference for the plasma discharge and as a return current path for the plasma power source. The emitted electrons ionize an inert process gas which is introduced within the chamber, and positive gas ions are formed creating a gas plasma discharge of high ion concentration. The positive ions of the plasma are attracted to the negatively-biased target and the ions thereby bombard the surface of the target, ejecting or sputtering atoms of the target material from the target. Atoms of sputtered material emitted from the target strike and adhere to the exposed surface of the substrate positioned opposite the target and thereby form a material film or layer on the substrate.
The sputtered target material travels in various directions from the target. While some of the sputter material contacts the substrate and deposits thereon, other material contacts other exposed surfaces in the processing space, such as the grounded surfaces of the chamber wall. The sputter material thus coats the chamber wall surfaces as well as the substrate surface.
Sputter deposition is used to deposit layers of various materials onto substrates and particularly to deposit dielectric material layers. However, when layers of dielectric material coat the metallic surface inside the chamber, the sputter deposition process is degraded. Herein, the term xe2x80x9cdielectricxe2x80x9d will be used to refer to materials conventionally referred to as dielectric, as well as semiconductor materials. More specifically, since the internal conductive surfaces of the chamber act as the ground reference for the plasma discharge and as a return path for the current flow of the plasma power source, any deposition of a dielectric material layer on the wall surfaces makes the plasma discharge unstable. That is, the high resistivity dielectric layer depositing or growing on the conductive wall surfaces of the chamber leaves the plasma discharge without a direct ground reference and changes the electrical impedance of the return current path of the plasma power source. An unstable plasma discharge results. Furthermore, the operation of the plasma power source and the process controls associated with the sputter deposition process also becomes unstable due to the lack of a sufficient ground reference and return current path in the chamber. The instability of the plasma discharge, in turn, causes an unacceptable deterioration in the deposited substrate layers and in the ultimate performance and properties of those substrate layers.
Unstable plasma discharges and the resulting degraded film properties have been observed when sputtering dielectric materials, such as lead zirconium titanate (PZT), barium strontium titanate (BST) and quartz, and also have been observed with the sputtering of semiconductor materials such as silicon. To address this problem in the past, the sputtering chamber and any associated internal elements and surfaces therein had to be periodically and regularly cleaned and replaced. The cleaning and replacement process is a time consuming and labor intensive procedure and results in higher production costs and reduced productivity and efficiency of the process.
Accordingly, it is an objective of the present invention to produce and maintain a stable plasma discharge during sputter deposition of dielectric layers on substrates.
It is another objective of the invention to reduce the deterioration of the sputter deposited dielectric layers associated with sputter deposition using prior art apparatuses and methods.
It is still another objective of the invention to provide for stable deposition of dielectric films while reducing time and labor costs associated with cleaning and maintaining the deposition chamber.
It is another objective of the invention to reduce production costs and increase the productivity and efficiency of the deposition process when sputter depositing dielectric films.
The above discussed objectives and drawbacks of the prior art are addressed by the present invention, which provides a stable plasma discharge for depositing dielectric layers on substrates.
The invention provides stable dielectric deposition and reduces the maintenance and replacement costs of the deposition chamber, thus reducing production costs and increasing the productivity and efficiency of the sputter deposition process.
The present invention comprises a sputtering chamber having a conductive internal wall surface which defines the processing space. A substrate mount is positioned in the processing space for supporting the substrate therein, and a sputter target mount is positioned for supporting a target of dielectric material opposite the substrate mount such that a layer of the target material may be sputter deposited onto the substrate. A plasma power source biases the target mount and target thereon, and excites a process gas introduced into the processing space into a plasma discharge to sputter material from the target. The wall surface of the chamber is grounded to provide a ground reference for the plasma.
In accordance with the principles of the invention, a conductive shield is positioned in the processing space between the target and the substrate, and is configured and positioned for capturing an amount of sputtered material which would deposit on the internal wall surface during the deposition process. The shield maintains a portion of the wall surface generally free from the extensive build up of dielectric material which occurs on the other unshielded surfaces. Therefore, the shielded wall portions are maintained in a generally conductive state to provide a stable ground reference for the plasma discharge and a stable return current path for the plasma source. The shield has a plurality of apertures formed therein which are configured to pass plasma therethrough during the deposition process to contact the stable ground reference and current return path provided by the shielded surface portions of the chamber wall. Thus, while a portion of the dielectric sputter material deposits on the chamber wall surface, the plasma still has a stable ground reference and return current path which is effectively accessed through the shield during the deposition.
More specifically, in the preferred embodiment of the invention, the chamber wall surface and conductive shield are coupled together and thus are both grounded. Therefore, the outer surface of the shield also acts as a stable ground reference. The shield is preferably cylindrically shaped with one end disposed proximate the target and another end disposed proximate the substrate. The shield surrounds a portion of the processing space between the target and substrate in which the plasma is located, and through which the sputtered target particles will travel. During sputter deposition, the inner shield surface, which faces inwardly into the processing space, and the side surfaces of the shield apertures intercept sputtered particles and become coated by the sputtered material. The outer surface of the shield, which faces toward the chamber wall surface, generally remains uncoated, as it collects very little or no deposition from the backwardly scattered particles of material which penetrate through the shield apertures to deposit on the wall surface. The shield effectively masks a portion of the wall surface to maintain that surface portion in a generally uncoated and conductive state to provide the stable ground reference for the plasma discharge. The uncoated wall portions and uncoated shield outer surface cumulatively provide the ground reference because the plasma contacts them through the apertures.
The plurality of apertures are arranged around the shield and are preferably geometric in shape, such as circular or rectangular. Minimal dimensions of the apertures, such as their diameter or width dimensions, are preferably between 0.25 and 2.0 times the length of the mean free path for the process gas, e.g., Argon. That is, the sputter deposition process gas, at a chosen process pressure, will have particles with a mean free path length between collisions, and the shield apertures are dimensioned based upon that length. It has been found that apertures with minimum dimensions in the range of 0.25 to 2.0 times the mean free path length of the process gas will pass enough of the plasma discharge to the chamber walls and outer shield surface to ensure a stable plasma an sputter deposition.
To further ensure a stable plasma, a suitable number of apertures must be utilized to ensure that a sufficient amount of plasma passes through the shield without having too much plasma between the shield outer surface and the chamber wall surface. To that end, the ratio of cumulative open aperture area to the overall surface area of the inner surface of the shield, or the xe2x80x9ctransparencyxe2x80x9d of the shield, is preferably chosen to be in the range of approximately 0.1 to 0.5. That is, approximately 10% to 50% of the shield is open for the plasma to pass therethrough. The disclosed transparency and aperture dimensions for the shield produce suitable deposition results without the plasma detrimentally etching the outer surface of the shield to any great extent.
The shield extends into the processing space between the target and substrate and has a cross section which is preferably similar to the shape of the target or substrate, that is, circular, to give the shield its overall cylindrical shape. While shielding the chamber walls, the shield is configured and positioned within the processing space to provide a generally unobstructed path between the target and the substrate so that the deposition rate for the substrate is not dramatically affected. The conductive shield is preferably treated to increase adhesion of the deposited material. For example, the shield may be grit-blasted or plasma-sprayed for increasing deposition adhesion thereto.
The invention produces a stable and continuous sputter deposition process for sputter deposition of dielectric material layers, and reduces the deterioration of the sputter deposited layers. The invention further reduces the maintenance and replacement of the deposition chamber and elements, and thus reduces production costs and increases the overall productivity and efficiency of the sputter deposition process. These benefits and advantages of the invention over the prior art apparatuses and methods will become more readily apparent from the Brief Description of the Drawings and Detailed Description of the Invention below.